1. Field of the Invention
The present invention relates to a power supply controller and power supply using the same, and more particularly, to a power supply controller with low cost, single integrated circuit (IC) chip, and high power factor, and a power supply using the same.
2. Description of the Prior Art
Power supply is an integral part for all electronic equipments, e.g. light emitting diode (LED) lighting. An ideal power supply should include power factor correction (PFC) to ensure that current and voltage waveforms are in phase to suppress undesired harmonics, so as to enhance power efficiency.
Power supply is also required to maintain the output voltage, current or power within a regulated range for efficient and safe operation of an electronic device. Feedback paths from an output of the power supply are used to control the output of the power supply within a specific range. Also, the prior art power supply usually adopts a separation of a primary-side from a secondary-side of a transformer for safety considerations.
Please refer to FIG. 1, which is a schematic diagram of a conventional power supply 10. The power supply 10 is a fly-back topology switching power converter and includes a transformer 100, a transistor 102, a pulse width modulation (PWM) control unit 104, a feedback control unit 106, a diode 108 (as a rectifier), and a capacitor C1. The transformer 100 includes a primary winding NP and a secondary winding NS. The feedback control unit 106 includes resistors R1-R4, a capacitor C2, a photocoupler 110 and a three-terminal shunt regulator 112.
The power converting function of the power supply 10 is realized via the PWM control unit 104 by controlling the transistor 102. The PWM control unit 104 generates a corresponding control signal VPWM to control the transistor 102 to be turned on or off according to a feedback signal VF from the feedback control unit 106. When the transistor 102 is turned on, electrical power is stored within the primary winding NP and the diode 108 is cut off due to the inverse bias voltage and the electrical power that the load of the power supply 10 requires is provided by the capacitor C1. When the transistor 102 is cut off, the electrical power stored within the primary winding NP transfers to the secondary winding NS, the diode 108 is turned on to transfer electrical power to the load. As can be seen from FIG. 1, the feedback signal VF is generated by the photocoupler 110 driven by the three-terminal shunt regulator 112. When an output voltage VOUT of the power supply 10 increases or decreases, the feedback signal VF changes with the output voltage VOUT and thereby changes the duty cycle of the control signal VPWM for adjusting electrical power outputted to the load to keep the output voltage VOUT stable. The three-terminal shunt regulator 112 needs peripherals including the resistors R1, R2, R3 and the capacitor C2 to complete the function. The resistors R1 and R2 are used for dividing the output voltage VOUT to generate the reference voltage of the three-terminal shunt regulator 112. The resistor R3 and the capacitor C2 are used for providing the loop comparison needed by the three-terminal shunt regulator 112.
However, such a secondary side feedback control mechanism increases circuit size and power dissipation, and utilizes the linear voltage regulator and the photocoupler to separate the primary-side from the secondary-side, which are both costly components. Furthermore, in order to cooperate with high power output applications, the whole driving circuit must use two independent controller ICs (integrated circuit), i.e., the PFC controller and the PWM controller. Thus, circuit designs not only become more complicated, but also more expensive.